1. Field of the Invention
The present invention relates to a method for determining the spatial coordinates of points as well as to a system for carrying out said method. The invention also relates to an optical device employed in said method.
2. Description of the Prior Art
The method finds applications in particular in high-precision topography as is required, for example, by owners or operators who exploit underground resources. In fact, in order to exploit or prospect mineral deposits, accurate measurements of subsidence around the zone considered are of great value since they provide indirect information on the evolutionary development of subsurface formations. These topographic measurements are also useful for seismologists in the analysis or prediction of tectonic movements.
In the article entitled "The Airborne Laser Ranging System, its capabilities and applications" (Tech. Memo. 83984, Sep. 1982), W. D. Kahn et al. disclose a telemetry system comprising an optical device mounted on board an aircraft and a plurality of retroreflectors placed on the ground. Starting from a subdivided laser beam, six telemeters are formed, each sub-beam being aimed at one of the ground reflectors through a porthole arranged in the floor of the aircraft. The aircraft is equipped with an on-board inertial system retimed by a radio aid system (Loran C or GPS) in order to determine the position of the aircraft and therefore to permit pointing of the telemeters at the targets as well as to determine its orientation since the six telemeters are not placed geometrically at the same location. Computation of the distances from the transit times of the laser pulses is performed in real time by a recursive Kalman filtering algorithm, thus entailing a long flight (several hours) and limiting the accuracy of measurements to the centimeter scale. The applications concerned are limited to measurement of deformation of the tectonic zones over short intervals of time.
This system involves considerable cost since the following costly elements are indispensable : an accurate inertial system, a high-performance on-board computer, a telemeter having six independent turrets for pointing at the targets, computation software packages, a large-size porthole having the requisite optical properties. Furthermore, the accuracy of these measurements is insufficient for following-up fine subsidences related, for example, to oil-field exploitation, especially on account of its limitation to six targets followed simultaneously. The reliability of the device is reduced by reason of the large number of mechanical elements mounted on board the aircraft. The preparation of each flight for aligning the inertial system is time-consuming. In addition, the aircraft is immobilized over long periods of time for the purpose of installing all the equipment required. Finally, the transmission of collimated laser beams towards the ground makes this system dangerous for the eyes of persons located in the field of vision of the aircraft.
The object of the invention is to overcome the disadvantages mentioned above by proposing a method and a system which make it possible to determine with very high precision the spatial coordinates of points located on the ground or in space and in particular to deduce therefrom the amplitude of minimal ground deformations at a large number of points located at the surface of the ground over an extensive zone while utilizing energy flux densities remote from laser safety threshold values.
Another object of the method in accordance with the invention lies in that fact that these measurements can be carried out rapidly, simply and with economical equipment, thus making it possible to overcome the principal limitations and constraints of known methods.